In a wire-cut electric discharge machine, it is necessary to supply a working fluid such as water, kerosene or the like into the working gap between the wire electrode for spark erosion and a workpiece in order to cause the electric discharge therein.
However, the width of this working gap is 0.05 mm or less and the diameter of the wire electrode is 0.2 to 0.3 mm or less, so that it is not easy to supply adequate working fluid into this working gap.
A conventional method is well known wherein a working fluid nozzle is used to produce the working fluid jet stream into the working gap where the spark erosion is presently performed along the wire electrode from the lower side or from both upper and lower sides of the workpiece.
The working fluid nozzle which was used in the early days is separately equipped from the guide device of the wire electrode, thereby causing the jet stream to cross the wire electrode. However, there are problems in that the reaching distance of the jet stream in the working gap is too short to completely fill the working gap by the working fluid and if the depth of the workpiece is 20 to 40 millimeters or more, this may cause the gaseous discharge, so that the spark erosion will be disabled.
To prevent this, a working fluid nozzle has been proposed wherein the wire electrode passes through the center of the opening and the jet stream is produced which overlaps the wire electrode and flows along its surface. This prior art has been disclosed in a Japanese Published Unexamined Patent Application Ser. No. 54538 of 1975.
However, this prior-art working fluid nozzle also has a problem in that the open air is swallowed into the working gap, so that it is impossible to perfectly prevent the occurrence of gaseous discharge. Therefore, a dual coaxial nozzle has been proposed which comprises a high pressure nozzle for producing a high pressure thin jet stream which flows along the surface of the wire electrode while keeping close contact therewith and a low pressure nozzle which is coaxially arranged outside of the high pressure nozzle and which produces a lower pressure jet stream. This dual coaxial nozzle has been disclosed in a Japanese Published Unexamined Utility Model Application Ser. No. 167496 of 1979.
Furthermore, in addition to the above-mentioned two applications, another system has been proposed wherein a workpiece is dipped into the working fluid and the working fluid is also discharge by a nozzle. Such system has been disclosed in a Japanese Published Unexamined Patent Application Ser. No. 20797 of 1972.
The objects of the above-mentioned improved nozzles are to produce a flow of the working fluid along the surface of the wire electrode in the working gap and to prevent the open air from entering the working gap.
Owing to these improvements, a certain sufficient amount of working fluid is supplied into the working gap and the occurrence of gaseous discharge can also be prevented.
However, even these well-known methods have problems in that the wire electrode is deformed into an arch shape due to the discharge pressure generated in the working gap, so that a short circuit may occur between the workpiece and the wire electrode and the work precision may be reduced especially in the places where working direction suddenly changes.
Although such a short circuit can be avoided by reducing the working speed at the turning point in the working direction, this countermeasure will in turn cause an over cut at that portion, namely the working gap will be widened; therefore, there is a problem in that the working precision will be adversely reduced.